To enjoin is to issue an urgent and official order. If the government tells loggers to stop cutting down trees, they are enjoining the loggers to stop.
Enjoin looks like it should mean bring together, and at one time, it did have that meaning. But in current usage, the only thing enjoin brings together is a command and the person on the receiving end of that order. If your doctor enjoins you to stop smoking, he is suggesting strongly that you quit.